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what happens during meiosis

Meiosis is a special kind of cell division that turns one diploid cell into four genetically different haploid sex cells (gametes) through two back‑to‑back divisions called meiosis I and meiosis II.

Big picture: what happens during meiosis?

  • DNA is copied once, then the cell divides twice.
  • Chromosome number is cut in half (diploid → haploid).
  • Homologous chromosomes pair up and exchange DNA (crossing over), creating genetic variation.
  • You end with four non‑identical gametes, each with one set of chromosomes.

A simple way to imagine it: first division separates pairs of chromosomes (meiosis I), second division separates copies (sister chromatids) of each chromosome (meiosis II).

Before meiosis: DNA is copied

Before meiosis actually starts, the cell goes through interphase.

  • In S phase, each chromosome is replicated, forming two identical sister chromatids held together at the centromere.
  • The cell is still diploid, but each chromosome is now double‑stranded (an “X” shape instead of a line).
  • Centrosomes (which build the spindle) also duplicate to prepare for division.

Meiosis I: the reduction division

Prophase I (pairing and crossing over)

This is the longest and most complex stage.

  • Chromatin condenses into visible chromosomes; each has two sister chromatids.
  • Homologous chromosomes (one from each parent) pair up side by side in a process called synapsis, forming tetrads (four chromatids together).
  • Crossing over (recombination) occurs: non‑sister chromatids swap DNA segments, producing new combinations of alleles.
  • The nuclear envelope breaks down and the meiotic spindle forms.

Crossing over is a key reason siblings look different from each other.

Metaphase I (pairs line up)

  • Homologous chromosome pairs line up along the cell’s equator as tetrads.
  • Spindle fibers attach to each homolog from opposite poles.
  • The orientation of each pair is random (independent assortment), creating even more genetic variation.

Anaphase I (homologs separate)

  • Spindle fibers shorten and pull each homologous chromosome to opposite poles; sister chromatids stay together.
  • This separation halves the chromosome number: each pole gets just one chromosome from each pair.

Telophase I and cytokinesis

  • Chromosomes may decondense slightly, and nuclear envelopes can briefly re‑form, depending on species.
  • Cytokinesis splits the cytoplasm into two cells.
  • Result: two haploid cells, each with chromosomes still in duplicated form (sister chromatids).

Some cells enter a short rest period called interkinesis between meiosis I and II, but no DNA replication occurs again.

Meiosis II: like mitosis, but on haploid cells

Meiosis II separates sister chromatids in each haploid cell, similar to a normal mitotic division.

Prophase II

  • In each haploid cell, chromosomes re‑condense if they had relaxed.
  • Nuclear envelopes break down again, and a new spindle apparatus forms.

Metaphase II

  • Chromosomes line up single file along the equator in each cell.
  • Spindle fibers from opposite poles attach to the centromeres of each chromosome.

Anaphase II

  • Centromeric cohesion is cleaved, and sister chromatids separate.
  • Once separated, each chromatid is considered an individual chromosome and is pulled to opposite poles.

Telophase II and cytokinesis

  • Chromosomes decondense and nuclear envelopes reform around each set.
  • Cytokinesis divides both cells, producing four total daughter cells.
  • Each daughter cell is haploid and genetically unique, ready to develop into sperm or egg cells.

Why meiosis matters (and what’s “special” about it)

  • It keeps chromosome number stable from generation to generation: fertilization restores diploid number when two haploid gametes fuse.
  • It creates genetic diversity through:
    • Crossing over in prophase I.
* Independent assortment of homologous pairs in metaphase I.
  • This diversity fuels evolution and helps populations adapt to changing environments.

A quick contrast with mitosis:

html

<table>
  <tr>
    <th>Feature</th>
    <th>Meiosis</th>
    <th>Mitosis</th>
  </tr>
  <tr>
    <td>Number of divisions</td>
    <td>Two (meiosis I and II)[web:1][web:3]</td>
    <td>One[web:1][web:9]</td>
  </tr>
  <tr>
    <td>Starting cell</td>
    <td>Diploid[web:1][web:3]</td>
    <td>Diploid (usually)[web:1][web:9]</td>
  </tr>
  <tr>
    <td>Ending cells</td>
    <td>Four haploid, genetically different[web:1][web:3][web:5]</td>
    <td>Two diploid, genetically identical[web:1][web:9]</td>
  </tr>
  <tr>
    <td>Main purpose</td>
    <td>Produce gametes for sexual reproduction[web:1][web:3][web:5]</td>
    <td>Growth, repair, asexual reproduction[web:1][web:9]</td>
  </tr>
  <tr>
    <td>Crossing over</td>
    <td>Yes, in prophase I[web:1][web:5]</td>
    <td>No[web:1]</td>
  </tr>
  <tr>
    <td>Genetic variation</td>
    <td>High[web:1][web:3][web:6]</td>
    <td>Very low[web:1][web:9]</td>
  </tr>
</table>

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